The advantages of typical two-stroke internal combustion engines, which include relatively higher power to weight ratio over a comparable displacement four-stroke engine and fewer moving parts, is offset by the disadvantages of total loss lubrication, with the inherent pollution of the burnt oil in combustion, and expensive roller bearings associated therewith.
Two-stroke engines typically cannot use a pressurised lubrication system and oil is added to the air/fuel mixture to allow lubrication of the piston within the cylinder.
Thus two-stroke engines burn oil, accounting for extra pollution. A lack of a pressurised lubrication system requires roller bearings on the crankshaft and con-rods which are able to operate in the oil/fuel mix, unlike cheaper and simpler slipper bearings. This requires a heavy and expensive crankshaft to be assembled around the roller-bearings.
Petrol Internal combustion engines typically also lose fuel efficiency, when throttled and at idle, due to the engine having to do work to overcome the “pumping” action of the piston on its up-stroke as it creates a partial vacuum down stream of the throttle valve. This is commonly referred to as pumping losses.
In paired cylinder engines, with the pairs of pistons arranged at 180 degrees apart (that is one piston is at top dead centre (TDC) when the other of the pair is at bottom dead centre (BDC), pumping losses from the vacuum down-stream of the inlet butterfly valve impact negatively on the fuel economy of the engine which must do work to overcome these losses.
Additionally, the common reed valve used as a check or one-way valve in the two-stroke induction cycle, has a tension resistance to be overcome, for it to be opened by the pressure differentials during induction. This increases resistance and thus flow, with reductions in efficiency, and is another contributor to pumping losses. Furthermore, the bulky reed valve block necessitates great cross sectional changes in the induction port, resulting in changes of charge velocity, in turn reducing charge, flow, power, efficiency, ram-effect and economy. The pivot valve described, in contrast, has no such tension loads to overcome, and maintains a greater consistency of inlet port cross sectional area, and hence flow speed, and improved ram effect.
It is an object of the present invention to address or at least ameliorate some of the above disadvantages.
Notes
    1. The term “comprising” (and grammatical variations thereof) is used in this specification in the inclusive sense of “having” or “including”, and not in the exclusive sense of “consisting only of”.    2. The above discussion of the prior art in the Background of the invention, is not an admission that any information discussed therein is citable prior art or part of the common general knowledge of persons skilled in the art in any country.